1. Field of the Invention
The present invention relates to intelligible reproduction of human speech or voice sounds, and more particularly, relates to systems for improving the intelligibility of voice sounds or signals that are degraded in some fashion, such as degradation caused by noise.
2. Description of the Related Art
Speech reproduction systems, such as public address systems, telephones, cellular telephones, two-way radios, broadcast radios, etc., are often used in environments where the listener hears the speech signal combined with noise. In some circumstances the noise is of such a level that intelligibility of the desired spoken communication from the speech reproduction system is greatly degraded.
A typical speech reproduction system includes a signal source that generates a speech signal, a loudspeaker, and a transmission system that carries the speech signal from the source to the loudspeaker. Typical signal sources include microphone, tape playback units, audio units, computer speech generators, etc. The types of noise in a typical speech reproduction system can be loosely categorized into three general groups based on the point where the noise enters the system, the noise groups include: source noise, transmission noise, and ambient noise. Source noise is noise introduced at the source. Wind noise in a microphone is an example of source noise. Transmission noise is noise introduced by the transmission system, that is, noise introduced between the source and the loudspeaker. A common example of transmission noise is the static that is sometimes heard in a telephone, cellular telephone, or radio broadcast. Ambient noise is noise present in the listener's environment, that is, acoustic noise that the listener hears in addition to the sounds from the loudspeaker. For example, the background noise heard in a noisy environment such as an airport or automobile is ambient noise.
There are many environments of this type where communication is lost, or at least partly lost, because the ambient noise level masks or distorts the speaker's voice, as it is heard by the listener. These environments include airports, subway, bus and railroad terminals, aircraft and trains, aircraft carriers, landing craft, helicopters, dock facilities, cars and other vehicles, and other noisy places. Few people who have attempted to understand a public announcement or use a telephone in a noisy airport can fail to appreciate the difficulty of extracting useful information in the presence of such ambient noise.
Attempts to minimize loss of intelligibility in the presence of noise have involved use of equalizers, clipping circuits, or simply increasing the volume of the sound from the loudspeaker system. Equalizers and clipping circuits may themselves increase the overall noise level, and thus fail to solve the problem. Simply increasing the overall level of sound from the loudspeaker does not significantly improve intelligibility and often causes other problems such as feedback and listener discomfort.